Age-dependent changes in the differentiated state of cells would be supported if evidence could be found for physico-chemical changes occuring in the chromatin. In addition, if cells of longer-lived mammalian species are inherently more stable in maintaining their proper state of differentiation as a function of time, then this characteristic may be reflected in some special or unique physico-chemical properties of the chromatin. To further examine these two possibilities, we have been measuring the ribosomal RNA gene dosage in mouse and primate species as a function of age and of maximum lifespan potential. Such changes in ribosomal RNA gene dosage appear probable in view of the apparent dynamic state of DNA and the presence of transposed elements (TE's), which could not only affect gene dosage number but also the chromosomal arrangement of genes. These changes could in principle alter the differentiated state of cells. Results to date indicate no age-dependent change in ribosomal RNA gene dosage in man or mouse, but more extensive and sensitive tests are being planned.